As the number of individuals having diabetes and similar medical conditions increases, self-monitoring of blood glucose levels has become a common practice. The purpose of monitoring the blood glucose level is to determine its concentration and then to take corrective action based upon whether the concentration is too high or too low, thereby bringing the blood glucose level back within a normal range. The failure to take corrective action can have serious medical implications, so daily self-monitoring of blood glucose is a fact of everyday life for diabetic individuals. Failure to test blood glucose levels properly and on a regular basis can result in serious diabetes-related complications, including cardiovascular disease, kidney disease, nerve damage and blindness.
Biosensors, such as test elements, are available to permit diabetic individuals to test glucose levels in a small blood sample. Common meter designs use disposable test elements that, in combination with the meter, electrochemically or optically measure the presence or concentration of glucose in the blood sample. The information typically is displayed as a blood glucose value and perhaps the time and date that the measurement was performed. This information is, in most cases, sufficient to allow diabetic individuals to adjust their dietary intake and/or insulin dosage, and in the case of low glucose values, may indicate a need for intake of sugar to avoid hypoglycemia.
Electrochemically measuring an analyte may be achieved by dosing a test element with a sample containing the analyte (e.g., glucose in an aqueous blood sample) to initiate a chain of reactions such as the chain shown below for glucose.
By applying a potential difference between working and counter electrodes of the test element, the reduced form of a mediator is converted to its oxidized form. A current associated with this reaction is proportional with a mass of reduced mediator, and consequently with the glucose concentration.
Exposing test elements, such as glucose test strips, to high humidity conditions, however, may cause the mediator to degrade to the reduced form or to other products that are electrochemically active in the same potential range. Additionally, high humidity may cause the enzyme to degrade to a reduced form, and the reduced enzyme may react with the mediator, producing mediator in its reduced form.
In this regard, electroactive degradation products can accumulate at the electrode surface, and a corresponding conversion under an applied potential will result in current generation even in the absence of a substrate (i.e., a blank current). This current can add to the current generated by the glucose reaction resulting in a positively biased result or increased strip failure rate.
The components of a test element's reagent matrix often are selected to dissolve quickly in an aqueous sample, thus providing a fast reaction time and a quick display of the result for the user. However, this provides the undesired consequence of making the reagent matrix susceptible to degradation by environmental humidity, thus compromising the shelf stability of disposable test elements.
Accordingly, methods of decreasing a test element's sensitivity to environmental conditions are desired. While test elements can be packaged and distributed in containers configured to prevent undesired ingress of moisture from the atmosphere or other sources, and while such test elements can be packaged in containers with desiccant materials incorporated within the container (such as in the lid or cap of the container) or in an insert that is left in the container with the test elements, known methods have not been effective for eliminating the risks of degradation.
For the foregoing reasons, there is a need for compositions and methods of improving the stability of test elements and test systems, and more particularly of glucose test strips, under unfavorable ambient measurement or environmental conditions, especially high humidity.